T139 Crustal Melt Flow in Orogenic Belts: Integrated Field, Microstructural, Geochemical and Geochronological Analysis of Migmatites and Associated Granites

Tuesday, 24 October 2006
8:00 AM - 12:00 PM

IN SITU GRANITOID GENESIS AND DEHYDRATION IN THE ADIRONDACKS

Paper number: T139-8
Program: Technical Sessions
Time: 9:45 AM – 10:00 AM

Authors

Abstract

The Adirondack Mountains, NY are ideal for studying melting and migmatites in conformable granitic leucosomes cutting melanocratic metasediments. Leucocratic bands range in thickness from mm- to dm-scale, and have average quartz:K-spar:plagioclase norms of 46:16:38 in the upper amphibolite facies NW Lowlands and 26:51:23 in the granulite facies SE Highlands. To determine whether these leucocratic bands were local melts of surrounding melanosomes or externally-derived intrusions, we have correlated cathodoluminescence (CL) imaging, in situ U-Pb geochronology by SHRIMP, in situ zircon δ18O measurements by CAMECA IMS 1280 ion microprobe, and metamorphic garnet δ18O measurements by laser fluorination at 9 locations. CL imaging indicates three populations of zircons in both regions: 1. relatively featureless rounded 'soccer balls' (metamorphic), and rhythmically zoned (igneous) cores truncated by either 2. discordantly zoned (igneous) or 3. unzoned (metamorphic) rims. The U-Pb ages confirm CL classification as either 'igneous' or 'metamorphic' and determine the timing of different events. Typical δ18O ion microprobe spot-to-spot reproducibility of zircon standards is ±0.14‰ (1SD). For zircons from 3 leucosomes from the NW, igneous cores average 7.7±2.2‰ (1SD, VSMOW, n=5), a single igneous rim is 8.2‰, and metamorphic rims and whole grains average 10.1±1.9‰ (n=14). In corresponding melanosomes, igneous zircon rims average 8.4±1.4‰ (n=3) and garnets average 10.5±0.5‰. Average zircon rim age is 1208±33Ma, while average metamorphic age is 1165±58Ma. For zircons from 6 leucosomes from the SE, igneous cores average 8.2±2.1‰ (n=7) and igneous rims average 11.6±0.6‰ (n=4); metamorphic rims and grains average 10.1±1.4‰ (n=36). In corresponding melanosomes, igneous zircon rims average 11.7±0.04‰ (n=2), while garnets average 11.5±1.4‰. Average zircon rim age is 1103±63Ma, while average metamorphic age is 1132±80Ma. Values of 'igneous' δ18O in leucocratic layers are unusually high for plutonic rocks, especially in the SE. These high δ18O values (>10‰) cannot represent nearby magmas and indicate melting of surrounding metapelites. Metamorphism and anatexis occurred concurrently, and dehydration by melting at 1.2-1.1Ga lead to low water activity during Ottawan granulite metamorphism at ~1050Ma.